The present invention relates to a material for gaskets in which a rubber layer is laminated on a one or each of opposite surfaces of a metal plate, and the gasket produced from the material has an advantage in that it is excellent in adhesion to fluids (an antifreeze and hot water) to be used in an engine cooling system of a car so that no layer separation is cause.
A rubber-laminated material for gaskets is formed in such a manner that a metal substrate formed, for example, of a carbon steel plate, a stainless steel plate, an aluminum alloy plate, a plated steel plate, or the like is coated with rubber and formed into a sheet. The thus obtained material is punched into predetermined shapes so as to be used as gaskets for various equipment.
FIG. 1 is a section showing the conventional rubber-laminated material for gaskets. The process of producing the illustrated material for gaskets will be described in the order of steps or producing. A substrate steel plate 1 is alkali-degreased, surface-roughed with a shot blast or a Scotch brush, and then phosphate-treated, so that chemical conversion treated layers 2 are formed. Chromic acid treatment is sometimes performed as the chemical conversion treatment for an aluminum alloy plate. Thereafter, coating of a primer mainly containing phenol resin (pure phenol resin or denatured phenol resin such as cresol denatured phenol resin or cashew denatured phenol resin) is performed to thereby form primer layers 3. Then, each of the primer layers 3 is coated to a predetermined thickness with a rubber solution prepared in such a manner that a carbon reinforcing filler, a generally-used vulcanizing agent, and an additive are added to nitrile rubber and the nitrile rubber is dissolved in toluene, an ester group solvent, and a ketone group solvent, and the rubber solution is dried and vulcanized, so that a rubber layer 4 is formed. The rubber layer 4 is coated, in accordance with a use condition of a gasket, with dispersion such as graphite, molybdenum disulfide, or the like, so that a non-adhesive layer 5 is formed. (Up to here, refer to the specification of U.S. Pat. No. 2,957,784.)
In the foregoing conventional rubber-laminated gasket, however, there has been caused a problem in that in the case of using the gasket in an engine cooling system of a car, the primer layers 3 of phenol group resin are invaded by an antifreeze or hot water so that the rubber layers 4 are separated from the metal 1, while the oil resistance and the Freon resistance are sufficient in the case of using the gasket in a refrigerator compressor.
The following five points are considered to be the reasons of the layer separation.
(I) Since a hydroxyl group (--OH) and a methylol group (--CH.sub.2 OH) of phenol resin which are the principal components of the primer layers 3 are hydrophilic, hot water penetrates into the primer layers 3 and the adhesion surfaces to thereby cause swelling of the phenol resin and reduction of the adhesive force of the surface, so that the separation is caused.
(II) Weak acid phenol resin is reaction-decomposed by an alkali rust preventive (an amine group or a phosphate) contained in an antifreeze, so that the separation is caused.
(III) Pores are formed in the rubber layers 4 by solvent vapor or a vulcanized gas generated when the rubber layers 4 are formed. Hot water or an antifreeze penetrates into the pores to thereby accelerate the reasons I and III.
(IV) Since the chemical conversion treated layers 2 are porous, water molecules are arranged or a moist metallic salt is formed on the metal surface to break the adhesion force between the metal and the primer.
(V) An anode-cathode reaction (an electrochemical reaction) is caused between a metal surface of a gasket end portion formed by punching and the porous, chemical conversion treated layers to thereby generate a hydrogen gas on the chemical conversion treated surface at the cathode side, so that the separation is caused.